The present invention relates to ultrasonotomography based on a Doppler signal from an object to be examined and more particularly to ultrasonotomography suitable for improving the performance of an ultrasonic diagnostic apparatus.
When an ultrasonic beam is applied at an incident angle to a blood flow inside a human body, the frequency of an ultrasonic wave reflected from the blood flow is shifted in accordance with velocity of the blood flow and a Doppler signal can be obtained. Thus, the blood flow velocity can be determined from this Doppler signal. Conventionally, in an ultrasonic diagnostic apparatus in which a Doppler signal is obtained to display a blood flow two-dimensionally, a blood flow in a tomogram plane from which a two-dimensional tomogram is obtained is measured and displayed. More particularly, where the array direction (azimuth direction) of transducers of a probe is y axis and a direction (elevation direction) orthogonal to the azimuth direction is x axis, focusing is effected in general in the elevation direction by means of a fixed lens (acoustic lens of fixed focus) and an ultrasonic beam is scanned in only the azimuth direction, whereby tomogram and Doppler data are obtained through scan in only the azimuth direction. One-dimensional electronic scan or mechanical scan is permitted by in order to effect two-dimensional scan, the operator manually moves a probe by changing the angle relative to x-axis direction to scan an ultrasonic beam broadly. Variable aperture in elevation direction and focus in elevation direction necessary for making a beam small in the x-axis direction have already been discussed in, for example, Ultrasonic B-scanner with multi-line array by D. Hassler, D. Honig and R. Schwarz, Medical division Siemens, Ultrasonic Imaging 4, pp. 32-43, 1982. Further, a method of applying this technique to obtaining Doppler data is described in JP-A-3-158144. Also, beam scan in the x-axis (elevation direction) is disclosed in, for example, JP-B-62-4988.